Nanocrystalline Spinel Mn Cu Fe O Ferrites—Synthesis and Structural Elucidation Using X-Ray Diffraction and Positron Annihilation Techniques

We have synthesized a series of nanocrystalline ferrite samples with the composition Mn_xCu_1-xFe₂O₄ (x=0.2, 0.4, 0.6, 0.8) by an advanced sol-gel auto-combustion method. The X-ray diffraction patterns confirm the existence of single-phase cubic spinel crystal structure of ferrites with lattice parameter ranges from 8.395 ?? to 8.45 ??. We report the equilibrium radii for tetrahedral and octahedral sites in the unit cells and the estimated cation distribution over the two sites of nanocrystalline Mn_xCu_1-xFe₂O₄. We also estimate the oxygen positional parameter as 0.389. The positron annihilation life time spectroscopic studies were carried out for all the samples and analyzed the variation of life time parameters ??₁; I₁, ??₂; I₂ and the mean life time ??_m to elucidate the defect structure of the nanocrystalline Mn_xCu_1-xFe₂O₄. We found that the overall vacant type defects fill up as the Mn²⁺ ion concentration, x, increases. The value of ??₁ varies from 150 ps to 170 ps and that of ??₂ varies from 295 ps to 335 ps, which are the characteristic values for nanocrystalline samples, indicating the presence of intergranular as well as surface-diffused vacancies in the crystal structure.